The present invention relates to torque transfer devices and, in particular, to fluid-actuated clutch and brake assemblies for the transfer of torque.
Torque transfer devices, such as friction clutches and brakes, are used in industrial applications to allow equipment to cycle more rapidly and/or more accurately than otherwise possible by using a selectively-operable motor alone. In particular, friction clutches are used to engage or disengage a load from the motor. Similarly, friction brakes are used to perform one of three basic functions: to stop the motion of a load, to hold a load stationary, or to slow a load. Torque transfer devices typically rely on either electromagnetism, pneumatic pressure, hydraulic pressure, or mechanical pressure, to either push or draw together adjacent pairs of friction surfaces to create a clamping force and, thereby, transfer torque between a pair of relatively rotatable members of the equipment. One important application of torque transfer devices is in winding and unwinding operations for delicate sheet material.
Conventional pneumatic and hydraulic torque transfer devices rely on fluid pressure to transfer rotary power between the pair of relatively rotatable members, mediated by the engagement of the friction surfaces. Specifically, fluid pressure is selectively applied to move a movable piston for urging the friction surface of one rotatable member into a frictional engagement with the friction surface of the other rotatable member. The fluid pressure is supplied to a sealed fluid communication space adjacent the moveable piston, which applies a force to actuate the piston. One common method of sealing the space is to provide a pair of spaced-apart O-ring seals carried by the body within which the piston moves. The O-rings compressively engage sealing surfaces on the exterior of the piston and, thereby, create a fluid-tight sliding seal. Another common method of sealing the enclosed space is to provide an annular sliding U-cup seal adjacent to the piston. The U-cup has side walls that slidingly move in contact with sealing surfaces in the enclosed space confining the U-cup.
Sliding fluid seals, such those provided by O-rings and U-cups, are susceptible to artifacts of static friction or stiction. Stiction is defined as the resistance to the start of motion and is usually quantified as the force required to overcome static friction and make two stationary materials slide against each other. Stiction exists when the value of the static friction exceeds the value of sliding friction between a seal and its corresponding sealing surface. O-rings experience significant stiction at their points of contact against the sealing surfaces. U-cups experience significant stiction where the inner and outer diameters of the side walls of the U-cup contact sealing surfaces. Stiction inhibits the movement of the fluid-actuated piston such that torque transfer devices have a poor sensitivity at low fluid pressures. Stiction causes the piston to suddenly lurch forward at the start of a stroke to engage the clutch or brake because the force needed to overcome static friction is significantly greater than the force of sliding friction experienced after motion is initiated. Stiction also causes the piston to have an intermittently jerky or jumpy movement during its travel as stiction is repeatedly experienced and overcome. Moreover, the presence of stiction may cause O-rings to distort and ultimately extrude from their glands. In particular, stiction is most perceptible when a low fluid pressure is applied to actuate the torque transfer device. Therefore, torque transfer devices employing O-rings and U-cups for actuation operate poorly at low fluid pressures.
Some conventional torque transfer devices utilize the inflation and deflation of a bladder or a tube for moving the piston. Bladders have minimal stiction because of the absence of sliding seals so that, in general, torque transfer devices actuated by bladders are sensitive to the application of a low fluid pressures. However, bladders are disadvantageous in that, as the components of the clutch or brake wear, the bladder cannot adjust to the increased dimensional tolerances so that the ability to transfer torque is lessened over the service life of the clutch or brake. Moreover, bladders require an air inlet hose to provide fluid pressure to the interior of the bladder which is difficult to route through the unit. Furthermore, bladders are prone to rupture at high fluid pressures.
Other conventional torque transfer devices apply a force to move the piston by utilizing an extensible diaphragm actuator having clamped peripheral edges. Each peripheral edge is secured by a two-piece frame or other similar clamping structure. Because diaphragm actuators lack sliding seals, torque transfer devices utilizing diaphragms have minimal stiction and are generally sensitive to the application of medium to high fluid pressures. However, a significant disadvantage of clamped diaphragm actutators is their relative insensitivity at low applied fluid pressures. Furthermore, the clamping of the peripheral edges limits the capability of the associated torque transfer device to accommodate the increased dimensional tolerances arising from wear. As the torque transfer device ages, its torque transfer ability degrades because the clamped peripheral edges of the diaphragm actuator cannot adjust or otherwise compensate for wear. Clamped diaphragms are typically dimensionally large so that the design of the torque transfer device must be dimensioned accordingly to accommodate the clamped diaphragm. Clamped diaphragms also present significant problems in fabrication and assembly that limit their applicability in torque transfer devices.
When the frictional surfaces of the torque transfer device are engaged, a significant amount of heat is generated. The heat energy is distributed through the torque transfer device by thermal conduction throughout the components of the torque transfer device. Typically, conventional torque transfer devices incorporate air passageways in the rotatable members to establish an air current to dissipate the heat energy by convection. However, despite the presence of the air current, the operation of the torque transfer device may be adversely affected because the ventilation afforded by the air passageways is inadequate to sufficiently reduce the operational temperature.
Accordingly, there is a need for an improved fluid actuator for torque transfer devices that permits operation with minimal stiction at low or high applied fluid pressures and that is simply incorporated into the design of torque transfer devices. Furthermore, there is a need for improved heat dissipation in a torque transfer device.
The present invention provides a torque transfer device that operates without perceptible stiction at both low and high fluid pressures for smooth rotary power transfers, that does not require routing an inlet hose through the device to provide fluid pressure, and that can compensate for increasing dimensional tolerances due to component wear. The torque transfer device of the present invention achieves these objectives by relying upon a diaphragm actuator that selectively urges a pair of relatively rotatable members into frictional engagement by applying a substantially linear force to a piston that urges the rotatable members into engagement. The diaphragm has a generally planar extensible web having an outer peripheral edges and an O-ring integral with the outer peripheral edge. The web is extensible by fluid pressure between an unextended and an extended state. The O-ring includes inner and outer aspects defined to respective sides of a plane bisecting the O-ring in a direction perpendicular to the web in the unextended state. The O-ring is received by a groove in a body member and has an inner aspect sealingly contacting an inclined wall of the groove. The body member also has a lip extending toward the web and over the groove so as to contact the O-ring along only its outer aspect in the unextended state of the web. As a result, the lip holds the O-ring in the groove while allowing the O-ring to sealingly move, such as by rotation and/or translation, therein.
In certain embodiments of the present invention, the web of the diaphragm actuator may be annular and may further include a second O-ring integral with an inner peripheral edge thereof. The second O-ring is positioned in a second groove disposed in the body member and utilizes a second lip for holding the second O-ring within the second groove while allowing the second O-ring to sealingly move therein.
By virtue of the foregoing, there is provided an improved diaphragm actuator for a torque transfer device well-suited to rotary power transfer applications requiring smooth motion at low or high fluid pressures and, in particular, for rotary power applications involving the motion of lightweight objects or delicate loads at either slow or high speeds. The diaphragm actuator of the present invention is self-contained and, as a result, the O-ring(s) are retained within the groove(s) without clamping so that the O-ring(s) may translate and rotate within the associated groove(s) while retaining a fluid-tight seal therewith. As a result, the diaphragm actuator can self-adjust to compensate for frictional wear of the friction disk and other moving components that alters dimensional tolerances between the components. To that end, each O-ring of the diaphragm actuator is moveably captured by a lip overhanging its groove, so that the torque transfer device has a reduced stiction at both low and high fluid pressures. The lip permits the O-ring to translate and rotate its groove while retaining a fluid-tight engagement.
In another aspect, the present invention provides structure for cooling a pair of relatively rotatable transfer members of a torque transfer device when the members are in an engaged condition for transferring torque. To that end, one of the pair of transfer members includes a plurality of radially-extending cooling holes, wherein each cooling hole has a substantially circular cross-sectional profile and a substantially uniform diameter along its length.
By virtue of the foregoing, the operating temperature of the pair of torque transfer members is lowered when the members are frictional engaged and, as a result, the occurrence of heat-related failures is significantly reduced and the operating lifetime of the torque transfer device is significantly increased.